1. Field of the Invention
The present invention relates to a method, by detecting a radio wave condition of the wireless LAN system ruled by standard of IEEE 802.11, to provide an optimum performance to a station under such condition.
2. Related Prior Art
Recently, one system becomes popular to construct a network through radio frequencies, which is typically represented by the wireless LAN system. This system uses license-free bands without official approvals, for instance, 2.4 GHz band for the 802.11b and 802.11g, while, 5 GHz band for 802.11a. Even the system uses those license-free bands, when each of stations freely begins its transmission, the data to be transmitted would be congested and the reliable communication would not be realized. Accordingly, the standard 802.11 rules simple management protocols below described.
That is, in the contention-based mode, the station with data to be transmitted, that is, the station to begin the transmission of the data first senses the carriers for a preset period, which is called as Distributed Inter Frame Space (hereafter denoted as DIFS) and is only capable of transmitting packets when the station does not sense the carriers, which means that channels are idle within a restricted area. When the station detects the carrier, which means that another transmitter station already occupies the carrier to communicate with another receiver station, the transmitter station senses the carriers again for another DIFS period after an idle time interval. Verifying a condition where the channels are vacant, setting the counter with a count created by the table of random numbers and down-counting the counter, the transmitter station may begin to transmit after the counter is reset. Deferred transmitter stations each sets the digit in respective down counter different from each other; accordingly, the next transmitter station may be randomly selected, which may avoid a situation where a specific transmitter station may transmit data, or always enters the waiting mode.
While, the receiver station communicating with the transmitter station returns, to the transmitter station after receiving the whole transmission data, an ACK when the whole data are correctively receiver but an NCK when an irregular reception is detected. When the NCK is replied, the transmitter station transmits the data again by reducing a modulation rate, the transmission speed, in a next acquired frame. One transaction terminates by the reply of the ACK/NCK, and another transmitter station, the same transmitter station may be assigned again depending on a random number set in transmitter station or a number of transmitter station that waits for the transaction, begins the next transaction after the period of DIFS. Thus, the 802.11 standard adopts a protocol called as the Collision Avoidance (CA) CSMA/CA that does not ensure the avoidance of the collision, where a plurality of transmitter stations begins the transaction at a time, but reduces the possibility of the collision.
However, the CSMA/CA protocol mentioned above, because it is a passive protocol dynamically between the transmitter station and the receiver station, various factors to degrade the transaction in an area covering the transmitter and receiver stations are occurred, which influences the radio wave condition between the stations and reduces the transaction efficiency. Typical factors are (1) a physical medium to affect the electromagnetic interference is placed between the transmitter station and the receiver station, (2) a plurality of transmitter stations locates within the area, and (3) the area is exposed to the radio wave output from the other transmitter station. The case (2) above is generally called as the “collision terminal” or “exposed terminal”, while the case (3) is often called as the “hidden terminal”.
The Japanese patent application published as JP-2007-166373A has disclosed a method to solve the subject “collision terminal”, while, a handshake protocol using RTS/CTS signal, which is optionally provided by the 802.11 standard, is known as a solution for the “hidden terminal” of the case (3) above.
The Japanese patent, JP-2004-007504A, has disclosed a method to establish the transaction, when one transmitter station is newly assigned in an area covered by a specific transmitter station, between two transmitter stations. Another Japanese patent application published as JP-2006-279253A has disclosed a method to avoid a data collision when the transmitter stations different from each other and having respective specific areas carries out the transaction with the same carrier frequency in an area overlapped with respective areas. Another Japanese patent published as JP-2008-042922 has disclosed a method to avoid remarkable degradation of the transaction throughput when the hidden terminal exits.
For the transmitter station and the receiver station, a transaction between which is influenced by the factors mentioned above, it is important to detect which is the current radio wave condition. The current radio wave condition for the transmitter station is not always difference same as that for the receiver station, while, the transmitter station is hard to know the radio wave condition of the receiver station. Adjusting the transaction mode, for instance, the output power of the radio wave and the data rate, depending on the current radio wave condition, it would be possible to perfume the transaction in a condition optimum to the current radio wave condition. It would be possible that the receiver station notifies the current radio wave condition to the transmitter station by the handshake mode, but wireless apparatuses widely used now generally do not implement such a protocol.
Moreover, the protocol mentioned above is premised that the transaction is carried out under a worse radio wave condition in the first place; the transaction of the information itself depends of the radio wave condition, which is hard to be acknowledged as a reliable method. Moreover, in an apartment house, multiple radio waves are congested from apparatus owned by different persons, which easily introduces the interference. The monitoring of the radio wave condition by an instrument such as spectrum analyzer is hard because the instrument itself is so expensive; and the transmitter station or the receiver station implements such a function would be inconsistent with the simplicity that the 802.11 standard aims.
Therefore, the present invention provides a method to estimate the current radio wave condition without appending another transmitter station or receiver station, and a configuration of the stations that implements the method.